Fuel delivery and control systems are deployed on aircraft to supply metered amounts of fuel to combustors associated with the aircraft's gas turbine engines. In a generalized fuel delivery and control system, a metering valve is fluidly coupled between a fuel source (e.g., one or more storage tanks) and the engine combustors. The metering valve includes a valve element (e.g., a piston) that is slidably mounted within a valve housing (e.g., a sleeve). The valve element is movable between an open position, a closed position, and various intermediate positions. The position of the valve element is adjusted by a valve actuator, which is, in turn, controlled by an engine controller. During operation of the fuel delivery and control system, the engine controller determines a desired flow rate through the metering valve and commands the valve actuator to adjust the position of the valve element to achieve the desired flow rate.
During flight, the temperature of the fuel conducted by a fuel delivery and control system may vary between, for example, approximately −46° Celsius (−50° Fahrenheit) to approximately 93° Celsius (200° Fahrenheit). As the fuel's temperature increases, the fuel's density decreases. If the fuel delivery and control system does not account for this change in fuel density, the system might not provide a consistent mass flow rate to the engine combustors over the operative temperature range. One known fuel delivery and control system regulates mass flow rate by employing a bypass valve that increases pressure upstream of the fuel metering valve as fuel temperature rises. The bypass valve is positioned downstream of a high pressure pump and is biased toward a closed position by a spring disposed within the bypass valve housing. The spring seats on a stack of bi-metallic discs. When heated by the fuel flowing through the bypass valve, the stack of bi-metallic discs expands and exerts a compressive force on the spring. The spring then exerts a greater bias force on the valve element, and less fuel is redirected back to the inlet of the high pressure pump. As a result, the pressure upstream of the fuel metering valve, and the volumetric flow through the metering valve, is increased. This increase in volumetric flow offsets the corresponding decrease in fuel density thereby maintaining a substantially consistent mass flow rate through the metering valve.
Although fuel delivery and control systems of the type described above are generally effective at regulating mass flow rate over an operative temperature range, such systems are limited in certain respects. For example, to achieve sufficient displacement of the bypass valve spring, an undesirably large number of bi-metallic discs may be needed. Certain fuel delivery and control systems have eliminated the need for such a stack of bi-metallic discs by continually monitoring fuel temperature and utilizing software to compensate for changes in fuel density; however, such systems require additional hardware components (e.g., a resistance temperature device).
It should thus be appreciated that it would be desirable to provide a fuel metering valve assembly that compensates for changes in fuel density over an operative temperature range that is reliable, lightweight, and relatively inexpensive to implement. Other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.